As one of video signal processes for improving picture quality in a television receiver, a DVD player, and the like there is frame rate conversion using motion compensation.
The principle of frame rate conversion will be described using FIGS. 1 to 3 with respect to a video signal captured by a camera for television broadcast (hereinbelow, referred to as a camera signal) and a video signal obtained by telecine converting a film to a television system (hereinbelow, called film signal or cinema signal).
FIG. 1(a) shows original frames A, B, C, and D of a camera signal of an NTSC system. In the case of converting the frame rate of the camera signal to 120 Hz, as shown in FIG. 1(b), an interpolation frame is added at a timing of every 1/120 sec between neighboring original frames (between the frames A and B, between the frames B and C, and between the frames C and D).
FIG. 2(a) shows original frames A, B, C, and D of a film signal which is telecine converted (2:2 pulldown) to the PAL system. Each of the original frames is repeated twice. In the case of converting the frame rate of the 2:2 pulldown film signal to 100 Hz, as shown in FIG. 2(b), three interpolation frames are added at 1/100 sec intervals between original frames neighboring at 25 Hz cycles (between the frames A and B, between frames B and C, and between frames C and D).
FIG. 3(a) shows original frames A, B, and C of a film signal which is telecine converted (3:2 pulldown) to the NTSC system. The odd-numbered original frames A and C are repeated three times, and the even-numbered original frame B is repeated twice. In the case of converting the frame rate of the 3:2 pulldown film signal to 120 Hz, as shown in FIG. 3(b), four interpolation frames are added at 1/120 sec intervals between original frames neighboring at 24 Hz cycles (between the frames A and B and between the frames B and C).
Each of the interpolation frames is generated by interpolating video images of an earlier original frame and a following original frame. The interpolation is performed by a method of calculating addresses of pixels of the earlier original frame and the following original frame used for calculating pixel values of an interpolation frame on the basis of parameters of an interpolation position of a video image in each interpolation frame and motion vectors between the earlier original frame and the following original frame, and then placing weights to the pixel values of the addresses in accordance with interpolation positions.
The frame rate conversion produces an effect of eliminating a motion blur in a camera signal and an effect of reducing a judder (unsmoothness of motion in a video image) in a film signal.
FIGS. 1 to 3 also show interpolation positions of video images in the interpolation frames in the conventional frame rate conversion. As shown in FIG. 1(b), the interpolation position of a video image in the interpolation frames, at the time of converting the frame rate of the NTSC camera signal to 120 Hz, is conventionally set to a position obtained by equally dividing the magnitude of the motion of a video image between the earlier original frame and the following original frame (the magnitude determined by motion vector detection) to two portions, that is, a position of 50% of the magnitude of the motion.
As shown in FIG. 2(b), the interpolation positions of video images in three interpolation frames at the time of converting the frame rate of the 2:2 pulldown film signal to 100 Hz are conventionally set to positions obtained by equally dividing the magnitude of video image motion between the earlier original frame and the following original frame to four portions, that is, positions of 25%, 50%, and 75% of the magnitude of the motion.
As shown in FIG. 3(b), the interpolation positions of video images in four interpolation frames at the time of converting the frame rate of the 3:2 pulldown film signal to 120 Hz are conventionally set to positions obtained by equally dividing the magnitude of the motion of a video image between the earlier original frame and the following original frame to five portions, that is, positions of 20%, 40%, 60%, and 80% of the magnitude of the motion.
FIG. 4 is a diagram showing examples of video images of the 3:2 pulldown film signal subjected to the frame rate conversion in the interpolation positions in the related art. The video image of an airplane moves between neighboring original frames A and B. In four interpolation frames, video images of the airplane are interpolated in positions obtained by equally dividing the magnitude of the motion into five portions.
In addition, for example, a technique related to such frame rate conversion is proposed in Patent document 1.
Patent document 1: Japanese Unexamined Patent Application Publication No. 2003-189257